Continuing this conversation with myself, I found two strands of beads that appear to match another variety on the "Traded for slaves" card - dainty, long faceted examples.

Karlis describes these as 3-layered hexagonal canes: transparent ultramarine, translucent bright Dutch blue, transparent ultramarine core. Measurements of 3 sample card beads are 21.3-21.4mm x 4.5-4.7mm.

My beads match the 3-layer description, measure in the 19-21 x 4-5mm.
range.

These beads don't appear to be too uncommon - I also have strands of them still strung on raffia, and they show up all the time on eBay, usually in close-up photos that of course unavoidably exaggerate their actual size.

I just thought it interesting that they seem likely to be in the mid-19th century ballpark as the larger "Russians."